-- "graceful fallback": mm components which don't have transparent- hugepage knowledge fall back to breaking a transparent hugepage and- working on the regular pages and their respective regular pmd/pte- mappings+- "graceful fallback": mm components which don't have transparent hugepage+ knowledge fall back to breaking huge pmd mapping into table of ptes and,+ if necessary, split a transparent hugepage. Therefore these components+ can continue working on the regular pages or regular pte mappings.

- if a hugepage allocation fails because of memory fragmentation, regular pages should be gracefully allocated instead and mixed in@@ -211,9 +211,18 @@ thp_collapse_alloc_failed is incremented if khugepaged found a range of pages that should be collapsed into one huge page but failed the allocation.

-thp_split is incremented every time a huge page is split into base+thp_split_page is incremented every time a huge page is split into base pages. This can happen for a variety of reasons but a common reason is that a huge page is old and is being reclaimed.+ This action implies splitting all PMD the page mapped with.++thp_split_page_failed is is incremented if kernel fails to split huge+ page. This can happen if the page was pinned by somebody.++thp_split_pmd is incremented every time a PMD split into table of PTEs.+ This can happen, for instance, when application calls mprotect() or+ munmap() on part of huge page. It doesn't split huge page, only+ page table entry.

thp_zero_page_alloc is incremented every time a huge zero page is successfully allocated. It includes allocations which where@@ -264,10 +273,8 @@ is complete, so they won't ever notice the fact the page is huge. But if any driver is going to mangle over the page structure of the tail page (like for checking page->mapping or other bits that are relevant for the head page and not the tail page), it should be updated to jump-to check head page instead (while serializing properly against-split_huge_page() to avoid the head and tail pages to disappear from-under it, see the futex code to see an example of that, hugetlbfs also-needed special handling in futex code for similar reasons).+to check head page instead. Taking reference on any head/tail page would+prevent page from being split by anyone.

NOTE: these aren't new constraints to the GUP API, and they match the same constrains that applies to hugetlbfs too, so any driver capable@@ -302,9 +309,9 @@ unaffected. libhugetlbfs will also work fine as usual. == Graceful fallback ==

Code walking pagetables but unware about huge pmds can simply call-split_huge_page_pmd(vma, addr, pmd) where the pmd is the one returned by+split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by pmd_offset. It's trivial to make the code transparent hugepage aware-by just grepping for "pmd_offset" and adding split_huge_page_pmd where+by just grepping for "pmd_offset" and adding split_huge_pmd where missing after pmd_offset returns the pmd. Thanks to the graceful fallback design, with a one liner change, you can avoid to write hundred if not thousand of lines of complex code to make your code@@ -313,7 +320,8 @@ hugepage aware. If you're not walking pagetables but you run into a physical hugepage but you can't handle it natively in your code, you can split it by calling split_huge_page(page). This is what the Linux VM does before-it tries to swapout the hugepage for example.+it tries to swapout the hugepage for example. split_huge_page() can fail+if the page is pinned and you must handle this correctly.

We want as much code as possible hugepage aware, as calling-split_huge_page() or split_huge_page_pmd() has a cost.+split_huge_page() or split_huge_pmd() has a cost.

To make pagetable walks huge pmd aware, all you need to do is to call pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the@@ -341,47 +349,80 @@ created from under you by khugepaged (khugepaged collapse_huge_page takes the mmap_sem in write mode in addition to the anon_vma lock). If pmd_trans_huge returns false, you just fallback in the old code paths. If instead pmd_trans_huge returns true, you have to take the-mm->page_table_lock and re-run pmd_trans_huge. Taking the-page_table_lock will prevent the huge pmd to be converted into a-regular pmd from under you (split_huge_page can run in parallel to the+page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the+page table lock will prevent the huge pmd to be converted into a+regular pmd from under you (split_huge_pmd can run in parallel to the pagetable walk). If the second pmd_trans_huge returns false, you-should just drop the page_table_lock and fallback to the old code as-before. Otherwise you should run pmd_trans_splitting on the pmd. In-case pmd_trans_splitting returns true, it means split_huge_page is-already in the middle of splitting the page. So if pmd_trans_splitting-returns true it's enough to drop the page_table_lock and call-wait_split_huge_page and then fallback the old code paths. You are-guaranteed by the time wait_split_huge_page returns, the pmd isn't-huge anymore. If pmd_trans_splitting returns false, you can proceed to-process the huge pmd and the hugepage natively. Once finished you can-drop the page_table_lock.--== compound_lock, get_user_pages and put_page ==+should just drop the page table lock and fallback to the old code as+before. Otherwise you can proceed to process the huge pmd and the+hugepage natively. Once finished you can drop the page table lock.++== Refcounts and transparent huge pages ==++Refcounting on THP is mostly consistent with refcounting on other compound+pages:++ - get_page()/put_page() and GUP operate in head page's ->_count.++ - ->_count in tail pages is always zero: get_page_unless_zero() never+ succeed on tail pages.++ - map/unmap of the pages with PTE entry increment/decrement ->_mapcount+ on relevant sub-page of the compound page.++ - map/unmap of the whole compound page accounted in compound_mapcount+ (stored in first tail page).++PageDoubleMap() indicates that ->_mapcount in all subpages is offset up by one.+This additional reference is required to get race-free detection of unmap of+subpages when we have them mapped with both PMDs and PTEs.++This is optimization required to lower overhead of per-subpage mapcount+tracking. The alternative is alter ->_mapcount in all subpages on each+map/unmap of the whole compound page.++We set PG_double_map when a PMD of the page got split for the first time,+but still have PMD mapping. The addtional references go away with last+compound_mapcount.

split_huge_page internally has to distribute the refcounts in the head-page to the tail pages before clearing all PG_head/tail bits from the-page structures. It can do that easily for refcounts taken by huge pmd-mappings. But the GUI API as created by hugetlbfs (that returns head-and tail pages if running get_user_pages on an address backed by any-hugepage), requires the refcount to be accounted on the tail pages and-not only in the head pages, if we want to be able to run-split_huge_page while there are gup pins established on any tail-page. Failure to be able to run split_huge_page if there's any gup pin-on any tail page, would mean having to split all hugepages upfront in-get_user_pages which is unacceptable as too many gup users are-performance critical and they must work natively on hugepages like-they work natively on hugetlbfs already (hugetlbfs is simpler because-hugetlbfs pages cannot be split so there wouldn't be requirement of-accounting the pins on the tail pages for hugetlbfs). If we wouldn't-account the gup refcounts on the tail pages during gup, we won't know-anymore which tail page is pinned by gup and which is not while we run-split_huge_page. But we still have to add the gup pin to the head page-too, to know when we can free the compound page in case it's never-split during its lifetime. That requires changing not just-get_page, but put_page as well so that when put_page runs on a tail-page (and only on a tail page) it will find its respective head page,-and then it will decrease the head page refcount in addition to the-tail page refcount. To obtain a head page reliably and to decrease its-refcount without race conditions, put_page has to serialize against-__split_huge_page_refcount using a special per-page lock called-compound_lock.+page to the tail pages before clearing all PG_head/tail bits from the page+structures. It can be done easily for refcounts taken by page table+entries. But we don't have enough information on how to distribute any+additional pins (i.e. from get_user_pages). split_huge_page() fails any+requests to split pinned huge page: it expects page count to be equal to+sum of mapcount of all sub-pages plus one (split_huge_page caller must+have reference for head page).++split_huge_page uses migration entries to stabilize page->_count and+page->_mapcount.++We safe against physical memory scanners too: the only legitimate way+scanner can get reference to a page is get_page_unless_zero().++All tail pages has zero ->_count until atomic_add(). It prevent scanner+from geting reference to tail page up to the point. After the atomic_add()+we don't care about ->_count value. We already known how many references+with should uncharge from head page.++For head page get_page_unless_zero() will succeed and we don't mind. It's+clear where reference should go after split: it will stay on head page.++Note that split_huge_pmd() doesn't have any limitation on refcounting:+pmd can be split at any point and never fails.++== Partial unmap and deferred_split_huge_page() ==++Unmapping part of THP (with munmap() or other way) is not going to free+memory immediately. Instead, we detect that a subpage of THP is not in use+in page_remove_rmap() and queue the THP for splitting if memory pressure+comes. Splitting will free up unused subpages.++Splitting the page right away is not an option due to locking context in+the place where we can detect partial unmap. It's also might be+counterproductive since in many cases partial unmap unmap happens during+exit(2) if an THP crosses VMA boundary.++Function deferred_split_huge_page() is used to queue page for splitting.+The splitting itself will happen when we get memory pressure via shrinker+interface.-- 2.5.0